The present invention relates to an electronic switch.
The use of electronic switches involves the difficulty that, because of the physical properties of the semiconductor devices used, e.g. C-MOS transistors, inductively or capacitively caused changes in the potential on an input may result in an abnormal switching behaviour of the switch. This is particularly undesirable if a plurality of switches are realized using integrated circuit technology, because other components will be disturbed, too, and if the switches are used as switching elements in switching networks, crosstalk and disturbing noise will occur.
In a C-MOS switch, which consists of the parallel combination of a P-MOS transistor and an N-MOS transistor in a p-well on a common substrate, it is required during normal operation that the input voltage lie in the range between the supply voltages of the two transistors. The bias used for the substrate of the P-MOS transistor is the positive supply voltage, and that for the substrate of the N-MOS transistor is the negative supply voltage, because otherwise the source-substrate junction of one of the transistors would be opened. The switching behaviour of the C-MOS switch becomes abnormal when one or both supply voltages fail, because then the source-substrate junction (s) is (are) no longer protected against being opened.
In addition, the channel of a transistor turns on when the negative (N-MOS) or positive (P-MOS) potential of the input exceeds the transistor's threshold voltage.
The object of the invention is to avoid malfunctions of the switch due to excessive (positive or negative) input voltages and in the absence of the bias voltage(s).